K. BAILEY

T H E INFLUENCE OF SHAKING UPON THE THIXOTROPIC SOL-GEL TRANSFORMATION.

BY WILFRIED HELLER AND HUGO L. ROEDER."

Received 24th March, 1942.

The " setting time " of a thixotropic system, 0, is the time between its liquefaction, by mechanical action, and its resetting upon standing at rest. A sample, kept in a test tube, is considered to have set if i t does not flow within a minute or so upon turning the tube upside down. A pre-requisite for useful setting time determinations is that the mechan- ical action shall not alter the thixotropic state. This factor has up to the present been neglected. Its investigation is the purpose of the present paper. The results apply primarily to setting time determina- tions by means of the " reversed tube " method, but they should be equally valid for the determination of 8 by other methods,l since these also involve liquefaction by mechanical action.

Results. The samples were poured into test tubes and shaken for varied lengths

of time in a machine which imitated closely the intensity and the frequency of a gentle shaking by hand. After shaking was completed, the samples were allowed to set. They were then liquefied, on briefly shaking by hand, and 8 was determined in the 'usual way. Each determination was re- peated several times so as to make sure of the approximate constancy of 8. Curve I of Fig. I gives results obtained with a V,O, sol prepared by Biltz's method and made thixotropic by adding a suitable amount of H,SO,. Shaking increased the setting time. The longer the samples were shaken the larger t9 became. This was not an effect of normal ageing during shaking. For, identical samples at rest showed no change of t9 during the same period of time. Curve I of Fig. z shows similar results for an iron oxide sol prepared by Graham's method and made thixotropic

*I The last known address of the co-author was : Phillips Gloelampenfabrieken, Eindhoven, Holland. The experiments were carried out at University College, London, England, during the summer of 1936. The authors wish to recall in deep gratitude the memory of H. Freundlich, in whose Department these experi- ments were carried out.

See, e.g., Pryce-Jones, J . Oil Colour Chem. Assoc., 1936, 19, 295 ; J . Scienti$c lnstr., 1941, 18, 39 ; Goodeve and Whitfield, Trans. Faraday SOC., 1938, 34, 511.

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192 THE THIXOTROPIC SOL-GEL TRANSFORMATION

by adding sufficient NaCl. The effect of shaking was less pronounced however ; after shaking for two hours 8 had increased 130 yo in the V,O, sol, but only 11 yo in the iron oxide sol.

In addition to the immediate effect of shaking upon 8 one observes an after-effect-

f shaken samples show an increase of 8 with time. This also is more pronounced the longer shaking has lasted. Re- sults are given in Curve I1 of Fig. I.

Discussion. Until recently,

o n l y “ t h i x o -

2 4 6 8 lo /2 were known to 7 ime of shaki;14 (h hours). b e s e n s i t i v e

1 I I I I I I I l a b i l e ” g e l s 2

According to t h e c u r v e s , liquefaction of t h i s o t r o p i c s y s t e m s o n brief shaking by hand must be repeated fre- quently before the effect of shaking upon 8 becomes notice- able, whereas in thixolabile s y s t e m s o n e brief shaking by hand leads to a measur- able change of 8. I t is known

FIG. I.--ln.uence of shaking u+on setting of tlzixotropic towards shak- v,o,-sols. ing. These differ

from thixotropic gels because the s e t t i n g t i m e

changes after each liquefaction until, after a certain number of settings and re-liquefactions, the system coagulates. The above experiments show that thixotropic systems do not behave in a fundamentally different manner and that there is only a quantitative difference.

I. Setting times immediately after shaking. 11. Setting times 2-4 days after shaking.

4 d 12 16 20 24 Eke of skskrirg (1;1 hours).

FIG. 2.-InfEuence of shaking upon setting of thixotropic iron oxide sols.

I. Setting times after shaking, in samples standing a t rest. 11. Setting times after shaking, in rotating samples.

that a thixotropic gel changes into a thixolabile one if its sensitivity is increased, for instance by adding more electrolyte or by changing the

2 Heller, Kolloid Z., 1930, 50, 125.

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W. HELLER AND H. L. ROEDER I93

PH in an appropriate way. Thus, if 0 curves are determined as a function of some stability factor in thixotropic systems i t is imperative to check whether or not &values remain constant upon repeated liquefaction. Otherwise one risks obtaining O-curves which in part only are a relative measure for the latent thixotropic state, another part including also the alteration of that state by the technique involved in measuring 8.

The alteration of 8 by shaking, long shaking in thixotropic systems and brief shaking in thixolabile ones, is likely to be caused by a " mechanical " coagulation, i . e . a coagulation brought about by shaking or stirring. I t proceeds only during, but not after, the mechanical treatment of the colloidal system. Most of the experiments on this type of coagulation were carried out by Freundlich and his c o - ~ o r k e r s . ~ It has become apparent that mechanical coagulation can be brought about in any sol provided that its stability is sufficiently In some- what hydrophobic sols, such*as those of iron oxide and V,O,, the critical stability below which shaking coagulates, seemed to be defined by a (-potential of about 5 0 mv. As the potential decreases further, the same shaking produces more coagulum, and the same amount of coagulum can be brought about by shorter and less intense shaking.

With possible exceptions-such as systems with strongly hydrated particles or with high colloid concentration-thixotropy, like mechanical coagulation, requires a sufficient reduction of stability of the sols. I t is for this reason that thixotropy is so closely related to coagulation5 and that a comparatively small increase in sensitivity of thixotropic systems leads to their coagulation. Thus, it is highly probable that by rendering a sol thixotropic it is at the same time rendered more apt to be coagulated mechanically. The natural stability of Biltz's V205 sols is already below the critical value which permits mechanical co- agulation. Obviously, therefore, they are subject to a still more pronounced mechanical coagulation after they are made thixotropic by adding sensitising electrolyte. Graham iron oxide sols may be so stable as to resist mechanical treatment unless sensitised. Nevertheless, if a large amount of electrolyte must be added in order to render them thixotropic, they may also become subject to mechanical c~agu la t ion .~

Mechanical coagulation leaves the (-potential unchanged 3C (Freundlich and Kroch, Z.C.). Its only known outstanding effect is the formation of irreversible, strongly anisometric aggregates. ti Probably, therefore, the increase of B during shaking is a consequence of the formation of these irreversible aggregates. Since the aggregates are strongly anisometric, the question arose as to whether or not they may lead to the appearance of rheopexy. The experiments carried out so far on this phenomenon7 leave no doubt that rheopexy is most pronounced in systems which contain strongly anisometric particles. Systems with rod-like or with plate-like particles may show rheopexy. In order to test our two systems for rheopexy we induced a gentle turbulent stream : by means of a special

(a) Freundlich and Recklinghausen, 2. physik. Chemie, A , 1.931, 157, 325 ; (b). Freundlich and Loebmann, ibid., 1927, 139, 368 : (c) Freundlich and Kroch, ibzd., 1926, 124, 155 ; ( d ) Freundlich and Basu, zbid., 1925, 115, 203.

Heller, Compt. rend., 1934, 198, 1776 ; 1934, 199, 354. Freundlich, Thixotropy, editions Hermann & Co., Pans, 1935. Heller, J . Physic. Chem., 1937, 41, 1041 ; Kolloid Beih, 1933, 39, I ;

Freundlich and Loebmann, 3b.

Freundlich and Juliusburger, Trans. Faraduy SOL, 1935, 31, 920 ; Julius- burger and Pirquet, ibid., 1936, 31, 445 ; Hauser and Reed, J . Physic. Chewz., 1937, 41, 911.

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I94 WOOD WATER RELATIONSHIPS

machine, the test tubes were rotated around their axis, the direction of rotation changing after each complete turn. For IOO rotations per minute, the V,O, sol showed very marked rheopexy, while the iron oxide sol showed rather the opposite effect, an increase in the setting time. As curve I1 in Fig. z shows, this opposite effect increased in magnitude the longer the mechanical treatment had lasted, i.e. the more anisometric aggregates had been formed. This seems to show that an anisometric shape of the particles is not a sufficient pre-requisite of rheopexy.

Summary. The setting time of thixotropic systems changes measurably after

shaking for a few minutes or a few hours, depending on the stability of the systems. The setting time of thixolabile systems changes after shaking for a few seconds. The alteration of the thixotropic and thixolabile state by shaking is attributed to a mechanical coagulation. It is advisable to take such a shaking effect into account whenever setting times are determined as a relative measure of the thixotropic state. The formation of strongly anisometric aggregates by shaking does not lead to rheopexy but to the opposite effect.

School of Chemistry, University of Minnesota,

Minneapolis, Minnesota.

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